The continuous production of ethyl alcohol has aroused considerable interest in recent years because it allows a further source of energy to be exploited and because it offers large-scale industrial units the advantages of uniform production, simpler control and a possible increase in productivity by avoiding cleaning and start-up phases. For the continuous fermentation of alcohol it is necessary that an adequate supply of sugar-bearing substrate, the so-called slurry be available to the microorganisms and that the extremely favourable fermentation conditions be maintained.
Traditionally, the commercial-scale production of ethyl alcohol by means of fermentation with yeasts has hitherto been using batch processes, i.e. the sugar-bearing substrate to be converted was placed in an aqueous solution in a fermenter, inoculated with starter yeast and left to ferment. During this period, the yeast microorganisms convert the sugar contained in the substrate to ethyl alcohol and carbon dioxide according to the following formula: EQU C.sub.6 H.sub.2 O.sub.6 =2C.sub.2 H.sub.5 OH+2CO.sub.2 +22 Kcal=100 kg. hexose=51.15 kg. ethanol+48.86 kg. CO.sub.2
The process is anaerobic and therefore the system does not require an air supply. When all of the sugar has been converted, the contents of the fermenter are harvested and subjected to distillation, during which the desired ethanol is obtained. Technical improvements, such as controlled addition of the starter yeast, maintaining the temperature at a constant level by cooling, controlling the pH, adding microbiological nutrients as well as mechanical mixing of the fermentation system resulted in an increase in yield and improved the conversion rate.
Because only small industrial units can be used for batch operation and uniform production cannot be achieved, there has been an increasing trend towards the continuous fermentation of aqueous slurries, and various processes have been developed for this purpose. It became more and more common to use processes in which the fermentation steps were separated with regard to space and time, in other words, the effect of the catalyst on the yeast on the one hand and the maintenance of the anaerobic yeast metabolism, i.e. yeast growth and reproduction on the other hand. This inevitably resulted in multi-stage processes. In the first stages the yeast cells reproduce with an oxygen surplus. The subsequent stages center around the anaerobic conversion of sugar to alcohol. A final stage aims at quantitatively converting all the residual substrate and obtaining a maximum alcohol concentration. It is unavoidable with such a process that one or other of the permissible limit parameters is exceeded or may not be met, thus damaging the biocatalyst and/or impairing the reaction.
In DE-AS No. 23 54 556 a continuous process is suggested in which alcohol and yeast biomass are produced alternately. In this process, the amount of slurry fed to the fermenter is controlled in such a way as to produce a concentration of fermentable substrate of max. 5 g/l in terms of glucose. If mainly alcohol is to be produced, the oxygen will be admixed in controlled quantities in such a way that 1 g. of yeast in the fermenter can consume 0.2-5 mg. oxygen/hour and when yeast biomass is produced the oxygen is fed at a rate of max. 400 mg/g yeast dry substance produced. This process exhibits a broad range of oxygen feed rates, i.e. up to a factor of 25, in fermentation and a high oxygen feed rate in yeast production. It is not guaranteed that the optimum oxygen concentration for fermentation is present in all process stages.